Refrigerators, freezers and air conditioners rely for their cooling ability on a vapor compression cycle in which a refrigerant liquid such as ammonia, various chlorofluorocarbons, Freon, or a combination of such refrigerants, is caused to evaporate by having the liquid absorb the heat from inside the refrigerator or freezer compartment or from inside the conditioned air space. Typical refrigerants evaporate at extremely low temperatures, and thus can potentially create subfreezing temperatures inside the refrigeration unit. The cooled refrigerant vapor is subsequently circulated through the compressor portion of the unit where both the pressure and temperature are raised, and then through a condenser, thus returning the refrigerant vapor to the liquid state.
As is well known, evaporator coils within refrigeration and freezer units are subject to the buildup of frost or ice. Frost forms when water vapor in the air condenses and deposits on the cooled surfaces of the evaporator coils due to the existence of a cold surface whose temperature is below both the dew point temperature and the freezing point. Ice forms due to a slow transformation of the frost layer into a more dense layer over time, due to a series of complex processes that may involve melting and refreezing of the frost layer and the subsequent seeping of the melt into the pores of the frost sublayers. Ice may also form if supersaturated air exists in the freezer which may result in deposition of these crystals on the coil surface due to a complex convection-dominated phenomenon. Accumulation of frost and ice decreases the efficiency of the evaporator, necessitating removal of the buildup on a periodic basis.
In order to maintain efficient operation of refrigeration units, methods and devices have been developed for removing frost from the cooling coils of evaporator units. In a conventional arrangement, a heating device is disposed proximate to the cooling coils and is used to periodically melt the accumulated ice or frost. The heater is generally controlled by a timer to initiate a defrost cycle at given intervals, in some cases determined electromechanically on the basis of accumulated compressor operating time. Other methods are also known for defrosting the evaporator coils. In all cases, overall energy efficiency and cooling capability of the unit are decreased substantially by the need to include a defrost cycle. In larger units such as walk-in coolers and freezers, automatic defrosting cycles can be impractically lengthy, thus requiring the unit to be completely shut down for defrosting.
For improved energy efficiency in refrigeration units, a need exists to minimize frost buildup on evaporator coils, reducing the necessity for, and duration of, defrost cycles and for shutdowns due to frost and ice buildup.